Prior art powered parachutes, powered hang gliders, and ultralight aircraft use conventional (free, open, or unducted) propellers which may be relatively inefficient, noisy, and unsafe. For the purposes of this applications, the term "light aircraft" is used to include powered parachutes, para-planes, powered hang gliders, ultralight aircraft, inflatable wing aircraft or the like. Although an unducted propeller may be suitable for relatively high speed aircraft (e.g., commercial aircraft or the like) an unducted propeller may lose efficiency at lower air speeds due to the effects of propeller tip vortices and the like.
Due to its inherent nature, a light airplane may operate a lower speeds (e.g., less than 30 miles per hour). According to Aerocrafter Homebuilt Aircraft Source Book Number 1, (Copyright 1993 BAI publishing 940 Adams St. Benicia, Calif. 94510), incorporated herein by reference, available powered parachutes have a top speed of 26 miles per hour (mph). The U.S. Federal Aviation Administration regulations have set the top speed of ultralight aircraft to 55 MPH. Thus, a parachute airplane (classified as an ultralight) or the like typically operates in the low speed regime.
The inefficiency of unducted conventional propellers may result in a need for higher powered engines, which may be heavy, expensive, and consume more fuel. In a parachute aircraft, cost and weight may be critical criteria, as a given parachute wing design may support a given design load. Increased engine sizes may thus reduce effective load capacity of a parachute airplane.
Moreover, as in most single engine aircraft designs, the effects of engine/propeller torque on the airframe may produce undesirable handling and control effects. In a conventional (e.g., rigid) aircraft, such effects may be countered with increased rudder action or trim. However, a parachute airplane may have an engine/airframe suspended from a parachute by parachute cords or the like. Thus, propeller torque may be more difficult to counteract, as the effect of the torque may be to twist the engine/airframe in relation to the parachute wing, resulting in difficulties in control and handling.
In addition to the aforementioned difficulties, an unducted propeller in a parachute aircraft may present a safety hazard to the operator or bystanders. By the nature of the design, the engine/propeller may be mounted relatively close to the operator, or as in some designs, onto the operator. The proximity of powerplant and propeller to the operator raises serious noise concerns. In addition, the operator and bystanders must exercise care to prevent injuries due to appendages being accidently placed in the path of the propeller. Moreover, due to the nature of the design of a parachute airplane, it is essential that parachute cords do not become entangled with the propeller either during take-off, flight or during landing.
A solution to the latter of these problems has been attempted by U.S. Pat. Nos. 4,657,207, 5,044576, and 5,160,100. These patents show aircraft with a parachute wing and propeller in a circular safety enclosure. U.S. Patent Number 4,657,207 discusses a circular shroud with support arms or the like to cover the propeller and to prevent accidental engagement or entanglement with the propeller. U.S. Pat. No. 5,160,100 discloses an enclosure called a "Guard Ring" for protection. Both patents disclose using a pair of counter-rotating propellers to provide thrust while counteracting propeller torque.
These circular safety shrouds, while providing some element of safety protection, guard only against accidental contact with the propeller from the edge of the propeller disc. Contact with the propeller from the forward or rearward portions remains relatively unprotected. Screens, grills, and the like have been attempted in some applications (e.g., backpack mounted engine/propeller) to safely enclose the propeller. However, such screens, grills or the like may reduce the efficiency of the propeller.
Inada, U.S. Pat. No. 5,044,576, issued Sep. 3, 1991, shows a propeller mounted in and enclosed by a frame. The frame is used to mount the propeller, fasten a parafoil wing, and carry the pilot. A drawing of the prior art included therein shows a propeller enclosed by a cart, but does not describe the function of the enclosure surrounding the propeller. Inada is directed toward a canopy withdrawing mechanism for,the parachute wing and does not address the problem of engine torque.
Means of compensating for the effects of torque reaction are addressed in Flynn U.S. Pat. No. 4,875,642. Flynn discloses an aircraft which decreases lift on one side by reducing the length from the body of the craft to the parafoil wing. However, such a technique may reduce the overall lift of the parafoil wing and may alter the handling of the para-plane. In addition, such adjustments, unless made continuously, may correct only for engine torque at a specified engine speed or thrust level. At other engine speeds, such adjustments in support and/or control cords may be insufficient or excessive.
Flynn also states "Compensation for the effect of torque is possible by the use of counter rotating propellers . . . Counter-rotating propeller arrangements are heavy and expensive." (Col. 1 line 58-59). Counter-rotating propellers (axial or twin) may be used to counteract the torque effect, however, such a technique adds additional weight to the aircraft, which, as discussed above, has a narrow design envelope with regard to total weight.
Ducted propeller aircraft and hovercraft are known in the art. For example; semi-rigid inflatable aircraft are known using ducted propellers mounted on a rotatable axes to provide thrust and lift. Hovercraft are known to use ducted propellers to provide efficient low speed thrust. A ducted propeller may be particularly efficient in the low speed regime. However, ducted propellers have generally not been applied to general aviation aircraft, ultralights, or para-planes.
One reason, as disclosed in Edgley, U.S. Pat. No. 4,544,115, issued Oct. 1, 1985 and incorporated herein by reference, may be that the flow of air from the duct over a fuselage or control and lift surfaces may reduce the efficiency of the ducted fan. (Edgley, Col. 1, lines 20-49). Edgley attempts to solve this problem by attaching the wings to the ducted propeller and or placing the ducted propeller behind the fuselage. While such a design may solve the efficiency problems noted by Edgely, the resultant design may be expensive to produce. In addition, in at least one embodiment, the tail control surfaces remain in the slipstream of the ducted propeller.
Even in the embodiment of Edgley, when the wing is mounted to the duct there still may be a slipstream effect between the duct and the wing. Air being drawn into the duct may divert airflow from over the wing, lessening the effectiveness of the wing for producing lift.
In addition to the above described difficulties in powerplant/propeller design, the parachute wing may present some difficulties to an operator. In order to achieve flight, the operator must accelerate forward to billow the parachute with air such that it assumes its parachute or airfoil shape. A prevailing wind may be helpful in this process, Upon takeoff and landing, therefore, the parachute wing may be dragged on the ground, at least momentarily, possibly incurring damage with ground obstructions. Moreover, such an arrangement requires a certain amount of space in order to take off and land without entangling the parachute in trees, shrubs and the like.
Inada, U.S. Pat. No. 5,044,576, issued Sep. 3, 1991, discussed above, does disclose a canopy withdrawing mechanism for the parachute wing upon landing. While such a system may prevent the parachute wing from being dragged on the ground, it add additional weight and complexity to the aircraft design.
Moreover, prior art parachute wing designs may present difficulties to the operator in initially inflating the wing on the ground using propeller blast, forward motion, or the like. In addition, inlet slots, openings, or the like, may be provided on a parachute wing to provide a passage for high pressure air to inflate the wing when the wing is in motion. Such slots, opening, or the like, may reduce the overall aerodynamic efficiency of the wing by increasing drag.
Thus, it remains a requirement in the art to provide a propulsion system for a parachute airplane which is lightweight, efficient, safe and provides thrust while reducing or eliminating propeller torque effects. In addition, it remains a requirement in the art to provide an aerodynamically efficient parachute airplane wing which may be readily erected and flown.